Rewrote timer code.

The new code is based on timestamps, rather than delays. This eliminates the
need to update every timer in timer_process(), which cost a non-trivial amount
of CPU time on low-end machines.

This involved changes to every user of the timer code, so this change almost
certainly introduces bugs.

src/pit.h

@@ -1,4 +1,4 @@
-extern double PITCONST;
+extern uint64_t PITCONST;
 void pit_init();
 void pit_ps2_init();
 void pit_reset();